Power amplifier having high heat dissipation

ABSTRACT

A power amplifier includes a substrate, a heat sink for dissipating heat, and a heterojunction bipolar transistor (HBT) disposed on the substrate. The HBT includes a collector, a base, and at least an emitter. The power amplifier further includes an emitter electrode directly connecting the heat sink and the emitter of the HBT. The emitter electrode is a flip-chip bump, and the heat sink is a metal layer that sandwiches the HBT with the substrate. Alternatively, the emitter electrode is a backside via that penetrates the substrate, and the heat sink is a metal layer, disposed on the substrate opposite the HBT.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 10/064,514, filedJul. 23, 2002, which is included in its entirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power amplifier, and morespecifically, to a heterojunction bipolar transistor (HBT) poweramplifier integrated circuit.

2. Description of the Prior Art

Power amplifier integrated circuits using HBTs are suitable for a widerange of applications and are particularly well adapted for use as highpower microwave amplifiers such as those used in mobile phones.

A frequent and often serious problem with HBT power amplifiers isexcessive heat buildup. Power amplifier integrated circuits operate athigh current density, and hence high power density, and thus, heatgenerated by devices of the HBT elevates junction temperaturesignificantly above ambient temperature. High junction temperaturedegrades the device reliability and limits the maximum power density ofthe device. Additionally, operating at higher power density risksthermal runaway of the power amplifier, in which the power amplifiersuffers catastrophic device failure. Furthermore, operating at a higherjunction temperature reduces device mean time to failure (MTTF).Typically, for a given application, larger devices are required toovercome this problem, leading to increased cost and inefficient use ofspace.

Adlerstein et al. in U.S. Pat. No. 5,986,324, which is incorporatedherein by reference, describes in detail an HBT structure and operationthereof. Miura et al. in U.S. Pat. No. 5,793,067, which is alsoincorporated herein by reference, teaches how a transistor structure canbe made with widened leads to reduce thermal resistance. However, bothAdlerstein et al. and Miura et al. teach the use of an emitterair-bridge that is costly and causes undue fabrication complexity.

The prior art heat dissipation in power amplifier transistors, such asHBTs, is inadequate. Moreover, prior art solutions providing heatdissipation are difficult and costly to manufacture, impacting yield andreliability. Such insufficient heat dissipation prevents prior art poweramplifiers from operating at high current densities or high power, anddictates a larger power amplifier for a given application. Finally, thecost associated with using the air-bridge for cooling is also muchhigher compared to a die without the air-bridge.

SUMMARY OF THE INVENTION

It is therefore a primary objective of the present invention to providea power amplifier integrated circuit having high heat dissipation tosolve the problems of the prior art.

Briefly summarized, the present invention includes a substrate, a heatsink for dissipating heat, a transistor disposed on the substrateincluding a collector, a base, and at least an emitter. The presentinvention further includes an emitter electrode directly connecting theheat sink and the emitter.

According to one preferred embodiment of the present invention, thetransistor is a heterojunction bipolar transistor (HBT).

According to one preferred embodiment of the present invention, theemitter electrode is a flip-chip bump and the heat sink is a metallayer, and the heat sink and the substrate sandwich the transistor.

According to another preferred embodiment of the present invention, theemitter electrode is a backside via penetrating the substrate and theheat sink is a metal layer, and the heat sink and the transistorsandwich the substrate.

It is an advantage of the present invention that heat accumulated in thetransistor is readily dissipated through the emitter electrode and theheat sink, such that the transistor can operate at a substantially highpower.

It is a further advantage of the present invention that the emitterelectrode directly connects the emitter to the heat sink in an efficientand cost saving manner.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of an HBT power amplifier according to thepreferred embodiment of the present invention.

FIG. 2 is a schematic diagram of the HBT power amplifier of FIG. 1dissipating heat.

FIG. 3 is a cross-sectional view of the HBT power amplifier of FIG. 1.

FIG. 4 is a schematic diagram of an HBT power amplifier according to asecond embodiment of the present invention.

FIG. 5 is a cross-sectional view of the HBT power amplifier of FIG. 4.

DETAILED DESCRIPTION

The present invention will be described in two embodiments. Bothembodiments comprise a heterojunction bipolar transistor (HBT) disposedon a GaAs substrate. However, the present invention is not limited bysuch and can be utilized with other types of transistors on differentsubstrates. Furthermore, the present invention should not be construedas limited to application as a power amplifier.

Please refer to FIG. 1. FIG. 1 shows a plan view of an HBT poweramplifier 10 according to the preferred embodiment of the presentinvention. Typically, HBTs comprise pluralities of bases, emitters,collectors, and other functional devices, however, the HBT poweramplifier 10 is shown as simplified for explanatory purposes. The HBT 10comprises a collector 12, an emitter 14 a, an enlarged emitter 14 b, anda base 16 all disposed on a substrate 50. The substrate 50 is a GaAssubstrate, but may be another suitable substrate material. The emitter14 a and the emitter 14 b are formed by a metallization process that iswell known in the art. Both emitters 14 a, 14 b are electrically andthermally connected together by another metallization layer 18. Theenlarged emitter 14 b is connected to a heat sink (item 22, FIG. 3) byan emitter electrode 20. In the preferred embodiment, the emitterelectrode 20 is a flip-chip bump that connects the enlarged emitter 14 bto the heat sink 22 for heat dissipation and electrical grounding. Theelectrical operation of the HBT 10 is well known in the art and will notbe described in detail in this description.

FIG. 2 shows the HBT power amplifier 10 during operation. Heat isgenerated in proportion to operating current density or power of the HBT10 and is accumulated at the emitters 14 a, 14 b. Heat as represented byarrow 40 flows from the emitters 14 a, 14 b to the flip-chip bump 20 andfinally to the heat sink 22. While some heat is radiated to surroundingcomponents and materials, a large portion of heat, as represented by thearrow 40, is thermally conducted through the efficient path provided bythe flip-chip bump 20 and the heat sink 22. In this way, the presentinvention provides enhanced cooling to functional devices of the HBTsuch as the emitters 14 a, 14 b.

Please refer to FIG. 3. FIG. 3 shows a cross-sectional view of the HBTpower amplifier 10 of FIG. 1 along a section line 3-3 shown in FIG. 1.In FIG. 3, the heat sink 22 is a metal layer. The flip-chip bump 20 isshown connecting the emitter 14 b and the metal layer 22. Heatrepresented by arrows 42 can be seen flowing from the emitters 14 a, 14b through the flip-chip bump 20 and finally to the metal layer 22. Theenhanced thermal conduction as provided by the preferred embodiment ofthe present invention allows the HBT power amplifier 10 to operate at asubstantially high power.

FIG. 4 shows a schematic diagram of an HBT power amplifier 10′ accordingto a second embodiment of the present invention. The HBT power amplifier10′ is similar to the HBT power amplifier 10 except that the HBT 10′comprises a second enlarged emitter 14 b rather than the emitter 14 a.The HBT power amplifier 10′ further differs in that emitter electrodes20′ are backside vias provided to both enlarged emitters 14 b. As aresult, heat dissipation from the emitters 14 b is evenly distributedbetween the emitters 14 b as represented by an arrow 44. Similar to thepreferred embodiment, the backside vias 20′ conduct the heat 44 to aheat sink (item 30, FIG. 5). The electrical operation of the HBT 10′ issubstantially the same as that of HBT 10. Furthermore, electricalgrounding of the emitters 14 b provided by the backside vias 20′ isessentially identical to the electrical grounding provided by theflip-chip bumps 20 in the preferred embodiment. A further difference ofthe second embodiment as shown in FIG. 4 is that the metallization layer18 is optional as emitters 14 b are both thermally connected andelectrically grounded to the heat sink 30. With this structure, thepresent invention provides enhanced cooling to functional devices of theHBT such as the emitters 14 b.

Please refer to FIG. 5. FIG. 5 is a cross-sectional view of the HBTpower amplifier 10′ of FIG. 4 along a section line 5-5 shown in FIG. 4.The heat sink 30 is a backside metal layer. As shown in FIG. 5, backsidevias 20′ penetrate the substrate 50. Heat is conducted from the emitters14 b through the backside vias 20′ and to the metal layer 30 asrepresented by arrows 46. The enhanced thermal conduction as provided bythe second embodiment of the present invention allows the HBT poweramplifier 10′ to operate at a substantially high power.

Naturally, the present invention as described in the preferredembodiment and the second embodiment, can be applied to an HBT poweramplifier having arrays of bases, emitters, collectors, and otherfunctional devices. Fabrication of the present invention HBT poweramplifiers 10, 10′ can be accomplished by currently availablesemiconductor manufacturing technologies.

Generally, the emitter areas are enlarged, as illustrated by emitters 14b, so that the flip-chip bump 20 or the backside via 20′ can be placeddepending on the specific application of the HBT amplifier 10, 10′. Anincreased amount of flip-chip bumps 20 or backside vias 20′ tends toincrease thermal efficiency at the expense of device area. Thus, aspecific layout to maximize thermal efficiency while minimizing devicearea is a design choice.

In contrast to the prior art, the present invention provides anefficient thermal path to the heat sink in the close proximity of thesource of heat generation, which is the emitter of the transistor. Theemitter electrode provides thermal conduction and electrical groundingto the emitter. The emitter electrode provided can be a flip-chip bumpor a backside via. The heat sink can be a metal layer that can bedirectly disposed on a substrate. For these reasons, the presentinvention provides a heat conduction path that is more thermallyefficient and more cost effective than that provided by the prior art.Accordingly, the present invention power amplifier can operate at ahigher current density and associated higher power than a comparableprior art power amplifier.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A power amplifier integrated circuit comprising: a planar substrate;a transistor integrally formed on the substrate, the transistorincluding a collector, a base, and an emitter, the emitter including anenlarged portion; a heat sink for dissipating heat; and an emitterelectrode electrically and thermally coupling the heat sink to theenlarged portion of the emitter; wherein in the plane of the substrate,the area of the enlarged portion of the emitter is greater than the areaof the emitter electrode, and the enlarged portion of the emitter has acenter of area located laterally away from the collector and the base byat least half a major dimension of the emitter electrode.
 2. The poweramplifier integrated circuit of claim 1 wherein the transistor is aheterojunction bipolar transistor (HBT).
 3. The power amplifierintegrated circuit of claim 1 wherein the emitter comprises ametallization layer.
 4. The power amplifier integrated circuit of claim1 wherein the emitter electrode comprises a flip-chip bump.
 5. The poweramplifier integrated circuit of claim 4 wherein the heat sink and thesubstrate sandwich the transistor.
 6. The power amplifier integratedcircuit of claim 1 wherein the emitter electrode comprises a backsidevia penetrating the substrate.
 7. The power amplifier integrated circuitof claim 6 wherein the heat sink and the transistor sandwich thesubstrate.
 8. The power amplifier integrated circuit of claim 1comprising more than one emitter, and emitters are mutually connected bya metallization layer.
 9. The power amplifier integrated circuit ofclaim 1 wherein the emitter electrode and the heat sink provide anelectrical ground connection to the emitter.
 10. The power amplifierintegrated circuit of claim 1 wherein the heat sink comprises a metallayer.
 11. The power amplifier integrated circuit of claim 1 wherein aplurality of transistors and a plurality of emitter electrodes aredisposed in an array, and operate as a functional device.
 12. The poweramplifier integrated circuit of claim 1 wherein the substrate is a GaAssubstrate.
 13. The power amplifier integrated circuit of claim 1 whereinthe emitter electrode directly connects the heat sink and the enlargedportion of the emitter.
 14. A method for manufacturing a heatdissipating power amplifier integrated circuit, the method comprising:providing a planar substrate; providing a heat sink for dissipatingheat; integrally forming a transistor on the substrate, the transistorcomprising a collector, a base, and an emitter having an enlargedportion located laterally away from the collector and the base; andelectrically and thermally coupling the heat sink to the enlargedportion of the emitter with an emitter electrode; wherein in the planeof the substrate, the area of the enlarged portion of the emitter isgreater than the area of the emitter electrode, and the enlarged portionof the emitter has a center of area located laterally away from thecollector and the base by at least half a major dimension of the emitterelectrode.
 15. The method of claim 14 wherein forming the transistorcomprises: disposing a metallization layer on the substrate to form theemitter; and disposing a second metallization layer to mutually connectemitters.
 16. The method of claim 14 further comprising: electricallygrounding the emitter through the emitter electrode and the heat sink.17. The method of claim 14 further comprising: arraying a plurality oftransistors and a plurality of emitter electrodes to form a functionaldevice.
 18. The method of claim 14 wherein the emitter electrodedirectly connects the heat sink and the enlarged portion of the emitter.19. A power amplifier integrated circuit coupling to a heat sink, thepower amplifier comprising: a substrate; a transistor integrally formedon the substrate, the transistor including a collector, a base, and anemitter, the emitter including an enlarged portion located laterallyaway from the collector and the base; an emitter electrode coupling theenlarged portion of the emitter to the heat sink.
 20. The poweramplifier integrated circuit of claim 19, wherein the area of theenlarged portion of the emitter is greater than the area of the emitterelectrode, and the enlarged portion of the emitter has a center of arealocated laterally away from the collector and the base by at least halfa major dimension of the emitter electrode.
 21. The power amplifierintegrated circuit of claim 20, wherein the emitter electrode comprisesa flip-chip bump or a backside via penetrating the substrate.